The invention relates generally to the field of cellular telecommunications networks and particularly to a method for optimizing paging cost in a cellular telecommunications network.
Typical cellular mobile networks use a radio frequency channel for wireless communication between a base station (stationary) and a mobile station (MS). The mobile radio frequency channel is a two-way communication channel. The channel consists of two frequencies with the channel bandwidth of 30 KHz. A number of radio channels are allocated to each cell site of a mobile switching system. These channels are logically divided into the voice channels and a single control channel. The voice channel carries the data and the voice between the mobile switching systems and the mobile stations. Each control channel carries the system control information such as paging message, mobile station registration, voice channel designation and hand off notification between the mobile switching system and the MS.
The control channel is divided into two different control channel formats: the forward control channel (FOCC) and the reverse control channel, based on the usage of the control channel. The FOCC is mainly used for delivering pages and system orders to the MS. The reverse control channel is used to transmit access requests from the MS to the mobile switching system coupled to a base station in the cell where the MS is located.
In many cellular networks, mobile switching center (MSC) does not have the exact information about the geographical location of all mobile stations operating in the network service area (SA). To locate a given MS, most of current mobile switching systems broadcast a page message through the FOCCs of all the cells in the SA. If the MS receives this page, it will acknowledge receipt thereof to the base station in the cell where the MS is currently located. The MSC is then notified as to the location of the MS.
A typical mobile switching system operating in large metropolitan areas consists of over 100 cells. In order to locate a mobile station, the mobile switching system has to broadcast the same paging message through all the FOCCs of over 100 cells in the system, but the mobile switch system receives only one reply message from the paged mobile station. Since the ratio of termination, response versus paging, is so low, the broadcasting of paging message to mobile stations causes a significant bottleneck problem in the FOCC resource. In order to alleviate the bottleneck generated by unnecessary paging messages sent through the FOCC, a number of location tracking strategies in a cellular mobile telephone network have been proposed.
There are three basic elements in a mobile switching system such as is illustrated in FIG. 1. The elements include a mobile station (MS), a base station, and a mobile switching center (MSC).
The mobile station (MS) could be a car phone, hand held, transportable, or any other type of wireless terminal. Each mobile station (MS) is identified by a mobile identification number (MIN). When the mobile station (MS) connects to the network, it transmits the MIN information, other data, and voice. It is also tunable on system command to a channel in the radio frequency spectrum allocated to the mobile switching system at certain power levels as pre-programmed.
Each of the base stations in the network are located at cell locations (the definition of a cell is given in the next paragraph) or sites throughout the mobile coverage area. Each base station receives and processes the radio frequency signals to make them suitable for the transmission between the wire line network and the radio network among all mobile stations interfacing with it.
The cell is a geographical area that the radio signal frequency, which is transmitted from a base station, covers to serve mobile telephone calls. In general, the size of a cell depends on the strength of the radio signal. The stronger the radio signal is, the wider area it can cover, and the larger the size of a cell is. The size of a cell can be adjusted by controlling the strength of radio signal that is transmitted from a base station.
The mobile switching center (MSC) operates as the central coordinator and controller for the mobile switching system. The MSC sends via a cell base station to the MS the different type of information such as the MSC's system identifier, the radio frequency that the MS should use for voice communication, page response acknowledgment, etc. The MSC also functions to provide the interface between the mobile station(s) and the public telephone network (PTN). In addition, the MSC performs the following functions:
1) Administration of radio channel allocation. PA1 2) Coordination of the grid of cell sites and moving mobile units. PA1 3) Maintenance of the integrity of the MSC as a whole. PA1 There are five repeating pairs following these two fields: the field of DOTTING and the field of WORD SYNC. Each pair contains two words, WORD A and WORD B, and each word with the size of 40 bits in length including a parity bit is sent out to each mobile station. The WORD A is for the MS whose least significant bit in the mobile identification number (MIN) is 0, and the WORD B is for the MS whose least significant bit in the MIN is 1. When an MS receives the FOCC message, it reads only one of these two words, either WORD A or WORD B, depending on its least significant bit of its MIN. PA1 All the repeated pairs of both WORD A and WORD B contain the same. The reason for sending repeated pairs of both Word A and Word B is for the purpose of transmission error checking. PA1 The format of WORD A and WORD B varies depending on the type of information that the FOCC message frame carries at any instant time. For example, if a FOCC message frame carries a paging message to locate a mobile station whose least significant bit of its MIN is 0, then the WORD A is formatted to contain a paging information for the mobile station. PA1 A busy/idle bit stream, which indicates the status of the reverse control channel, is inserted in the message frame. The reverse control channel is busy if this bit is set to 0, otherwise it is 1 indicating that the reverse control channel is free. First, this bit occurs at the beginning of each sequence of both dotting and synch. Then, it also appears at the beginning of the first repeat of WORD A, and it is also inserted into each word at the distance of every 10 bits.
The mobile switching system is organized in the hierarchical manner. As shown in FIG. 1, the MSC is connected to the public telephone network (PTN). Each MSC typically controls about 100 base stations on an average, and each base station serves a number of mobile stations simultaneously.
In general, a mobile station (the origination) can originate a call from any one of cells to any telecommunications device (the termination) in any geographical location area. As soon as the MSC receives a request from the originating mobile station MS, it determines the location of the mobile station and allocates a voice channel available to the mobile user (originator). Once the voice channel is allocated to the originator, the MSC connects to the teminating device. Where the terminating device is another mobile station MS, the MSC pages the terminating (terminal) mobile, and the MSC connects the call if the paged mobile responds to the paging.
The forward control channel message format, which is used between the base station and the MS, varies depending on the switching system standard utilized thereby. For example, in North American there are several Mobile Switching System Standards including the Electronic Industry Association Interim Standards known as IS-54 and IS-136 In this section, the FOCC message format of IS-54 is illustrated.
The forward control channel (FOCC) is a continuous data stream sent at the rate of 10 kbps (kilobits per second) from the base station to all of the mobile stations to each MS coupled to the cell transmitting the FOCC. The message format of the FOCC is described as follows:
As shown in FIG. 2, the FOCC starts with two fields, the field of DOTTING with ten bits in length and the field of WORD SYNC with eleven bits. Both fields are sent to the mobile station (MS) to synchronize with the incoming data.
Each FOCC message frame consists of total 463 bits at the transmitting rate of 21.598 message frames per second. Accordingly, the number of paging messages that a FOCC can carry is limited by the bandwidth of the FOCC. However, the number of page messages that the system will attempt to initiate is call initiation dependent regardless which forward control channel format is used. In other words, as the number of call attempts increases in the system, the number of page messages in the system increases correspondingly thereby reducing the capacity of the FOCC to perform all its functions including paging.
Since the bandwidth bottleneck of a FOCC is due to heavy consumption and high occupation rate by paging, a number of mobile location tracking and paging methods have been studied in order to reduce the FOCC consumption in locating a paged mobile station.
An aggregate mobile station tracking strategy and reporting center approach was proposed by A. Bar-Noy, and I. Kessler, in "Tracking Users in Wireless Communications networks," IEEE Infocom Conference on Computer Communications, pp. 577-584, June 1994. This method designates a subset of all the cells in a service area as reporting centers. The strategy is described as follows: a mobile station sends the location update message whenever it enters a new reporting center and a tracking for the mobile station is restricted to the set of adjacent cells of the reporting center. This method is not efficient because for two major reasons. First, if a mobile station does not move far and it moves in and out of the reporting center frequently, the frequency channels will be heavily loaded due to unnecessary paging activities for location update. Second, even though a MS may move around frequently, it may never report changes of its current location for a long time if it never enters to a reporting center. Also, this strategy can not guarantee that the mobile station will responsed to the paging message sent to the adjacent cells of the last visited reporting center.
A location area approach was presented by S. Madhavapeddy and K. Basu in "Optimal Paging in Cellular Mobile Telephone Systems," Proceedings of the 14th International Teletraffic Congress, pp 493-502, June 1994. In this approach the mobile service area is partitioned into equal sized location areas. Each location area consists of a number of cells. All mobile stations register whenever they cross over from one location area to another. This strategy keeps track of the precise location area of every mobile station, and it also guarantees a mobile station can be exactly located whenever the MSC pages the most recent cells in the location area registered by the mobile station, but it is still inefficient for the following reasons. First, although the paging activities are reduced to only these cells in the most recently registered location area, all the registrations must be performed in the cells that are along the borders of the location areas where the mobile was most recently located. In addition, this approach makes the reverse control channels for these cells heavily loaded and congested. Second, all cells in the location area are still paged so that the call termination rate per cell paged is very low.
An individual mobile station tracking strategy utilizing the mobile station's mobility pattern was proposed S. Tabbane in "An Alternative Strategy for Location Tracking," IEEE Selected Areas in Communications, pp 880-892, June 1995. This strategy partitions the service area into several location areas based on each mobile station's mobility pattern. The location areas are arranged in descending order of location probabilities of mobile stations. When the MSC tries to locate a mobile station, the MSC pages the mobile station in these cells of location area with the highest probability first. If the mobile station is not found in the first paged location area, it will page the next highest one, and so on. The MSC will not stop paging until it receives a response message from the paged mobile station. However, this approach only considers the mobile probability pattern for the entire location area but not for each individual cell.
Another approach is described in a copending patent application entitled "Method for Reducing Paging Load In A Cellular Communication Stytem", Ser. No. 08/743,689, filed on Nov. 6, 1996, the content of which is incorporated herein by reference. The approach taken therein is primarily to determine a polling scheme based on the collective polling history of all the mobiles operating within the service area served by the network during a given monitoring period. As a result, this approach is not optimized on a subscriber by subscriber basis and results in a relatively high paging cost.